1. Field of the Invention
This invention relates to an apparatus and method for developing an electrostatic latent image using a high resistance magnetic developer.
2. Description of the Prior Art
Dry developing methods include a so-called "two-component developing method" using a toner and a carrier and a so-called "one-component developing method" not using the carrier.
Conventional one-component developing methods generally use a magnetic brush developing method. In this method, fogging is likely to occur on the picture image because both positive and negative polarities are dispersed in the magnetic developer due to frictional charging and the developer attaches to non-image areas due to the frictional charge or image force when the developer comes into contact with a picture image receptor.
It has conventionally been attempted to prevent fogging by applying an electric bias to the sleeve of the developing machine, but mere application of a d.c. voltage is not effective, and, on the contrary, increases the fogging at times because of the variance of the amount of charge in the developer itself.
In a developing method which develops the latent image by applying a high a.c. voltage to make the developer flight, fringe-like fogging is likely to occur around the picture image because not only developer having the desired polarity but also developer having an unnecessary polarity are brought into the developing zone. Another problem is that the latitude is very narrow with respect to photosensitive materials having a high residual potential.
Thus, the principal problem with the magnetic brush developing method using a high resistance magnetic developer lies in that fogging is likely to occur. The primary cause of this problem is that the quantity of charge in the developer due to frictional charging is unstable on both polarities. To solve this problem, a charge injection method or the like has been proposed but it has been very difficult to control the amount of charge.
As a method of applying electric bias, it has been a customary practice to apply a voltage close to the potential of non-image areas of the picture image receptor on the developing device, but since the latitude is narrow in the high resistance magnetic developer, applying a bias invites fogging or lowers the picture image density.
FIG. 1 illustrates the state in which a toner layer 3 on a developing sleeve 2 is in contact with a picture image receptor 1. When a d.c. voltage is applied to the developing sleeve 2, the developer attaches to the picture image receptor 1 in the amounts illustrated in FIG. 2. In this drawing, V.sub.R represents the voltage of non-image areas, with the potential of the electrostatic latent image areas being positive. When the applied voltage is changed in both the positive and negative directions, the quantity deposited shows a minimum near V.sub.R in the non-image area but fogging occurs on the positive side with respect to V.sub.R as positive toner attaches, and on the negative side with respect to V.sub.R as negative toner similarly attaches. This phenomenon always occurs with the one-component magnetic developer.
The simplest method of eliminating fogging is to prevent the developer layer from coming into contact with the picture image receptor. In this case, however, the voltage at the image area must be increased and the toner layer on the developing sleeve must be made uniform in order to increase the picture image density and to obtain a uniform picture image. Also, the toner layer must be thin. However, the resulting picture image is not clear.